Electrolytic codeposition of copper with fine particles

ABSTRACT

FINE PARTICLES OF NON-CONDUCTING MATERIALS DO NOT CODEPOSIT READILY FROM AQUEOUS ACIDIC COPPER PLATING BATHS, UNLESS THERE IS PRESENT IN THE BATH MONOVALENT CATIONS SUCH AS THALLIUM, AMMONIUM AND THE ALKALI METAL CATIONS. THESE MONOVALENT CATIONS ARE ESPECIALLY EFFECTIVE IN ACIDIC COPPER SULFATE PLATING BATHS FOR THE CODEPOSITION OF DISPERSED FINE, BATH-INSOLUBLE, NON-CONDUCTING PARTICLES SUCH AS BARIUM SULFATE, ZIRCONIUM OXIDE, ETC. POSSIBLE ENGINEERING APPLICATIONS FOR SUCH 2-PHASE COMPOSITE PLATES ARE IN ANTI-SEIZING AND IMPROVED WEAR RESISTANCE.

United States Patent 3,672,970 ELECTROLYTIC CODEPOSITION OF COPPER WITH FINE PARTICLES Thaddeus W. Tomaszewski, Dearborn, Mich., assignor to The Udylite Corporation, Warren, Mich. No Drawing. Filed June 19, 1969, Ser. No. 834,868 Int. Cl. C23h 5/48, 13/00 US. Cl. 204-52 R 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the cathodic codeposition of multitudinous fine particles of bath-insoluble inorganic and organic powders dispersed in aqueous acidic copper plating baths. More particularly this invention provides a means to improve and uniformly increase the degree of codeposition of bath-dispersed particles throughout the electrodeposited copper plate.

The densely codeposited inorganic fine particles in the matrix of the electrodeposited copper can increase the tensile strength of the copper and its resistance to high temperature creep, and the inorganic and organic particles can greatly decrease the tendency for copper surfaces to stick and seize, yet without causing any appreciable loss in electrical or heat conductivity.

Thus, these copper deposits can be used for various engineering purposes either as a composite copper plate on top of a basis metal, including wire or strip, or as an electroform.

It has now been found that bathsoluble salts of monovalent cations when present in the standard acidic copper plating baths of either low or high concentrations of copper ions make possible or greatly increase the degree of codeposition of dispersed fine bath-insoluble inorganic particles, such as barium sulfate, strontium sulfate, aluminum oxide, titanium oxide, kaolin (a hydrous aluminum silicate), rare earth fluorides, stannic oxide, finely powdered glass, lead sulfate, lead phosphate, ceric oxide, boron nitride, graphite, molybdenum sulfide, iron silicide, silicon carbide, boron carbide, boron, silicon, silicon dioxide, PVC, nylon, Saran, etc.

Of the monovalent cations, thallium, cesium and rubid ium are outstanding in the promotion of the codeposition of the fine inorganic bath-insoluble powders, as well as the codeposition of organic resin powders. As little as 0.05 gram/liter of thallium ion is effective in promoting the codeposition of the bath-dispersed (by air agitation or mechanical agitation) fine powders. The presence of 0.05 g./l. of thallium ion is as effective as about 0.1 g./l. of cesium ion and 0.3 g./l. of rubidium ion, and these three ions are surprisingly effective at these low concentrations. For ammonium, potassium, sodium and lithium, higher concentrations of these ions are needed to approach the effectiveness of the low concentrations of thallium, cesium and rubidium. In fact it takes at least 3 g./l. of the ammonium ion, and even more of sodium and potassium ions to about equal the effect of 0.05 g./l. of thallium ion. The silver ion is not as desirable as the other monovalent cations because it precipitates the traces of chloride ions (5 to about 100 mg./l.) necessary for the best copper deposits from the standpoint of smoothness, and of brightening when organic brightening additives are also used in the acid copper plating baths. It is for this reason as well as its immersion problems, that it is not a preferred monovalent cation.

The salts of these monovalent cations may be carbonates, sulfates, sulfonates, acetates, tartrates, fluoborates, fluosilicates, hydroxides, benzoates, sulfides, fluorides, etc. With chlorides it is essential that the concentration of the halide ion is kept below the value (about 200 m g./l.) that will cause the formation of a thick insulating film of cuprous chloride on the anode as well as on the cathode during electrolysis. It is interesting to note that chloride salts in the low concentrations necessary to avoid the formation of a visible cuprous chloride film on the cathode and anode, are more effective than the fluoride salts which usually necessitate higher though not excessive, concentrations to promote the codeposition of the dispersed powders. Thus the anion other than the main anion of the copper salt also has an effect on codeposition. For example, with 0.3 g./l. of ammonium chloride present in an air agitated acid copper sulfate electroplating bath and containing the same concentration of dispersed fine barium sulfate powder, a higher percentage of codeposited barium sulfate particles was obtained than with 20 g./l. of ammonium fluoride. Addition of 0.3 g./l. of ammonium chloride promoted greater codeposition of barium sulfate than did 30 g./l. of sodium sulfate. However, it is the monovalent cation that is the key to the most effective fine particle codeposition from the sulfate bath, and of the monovalent cations the most effective by far were the thallium, cesium and rubidium cations, either used alone or in combinations.

It is best to add the thallium, cesium and rubidium ions in the form of sulfates or fluorides. Added as the chloride in a concentration around 1 g./l., the formation of cuprous chloride on the copper anodes will cause excessive polarization making it ditficult to draw current with the usual plating voltages. If lead anodes are used in such acid copper sulfate plating solutions, the formation of cuprous chloride is avoided at the anode, though it occurs at the cathode with this small but nevertheless excessively high chloride concentration.

The bath-insoluble fine powders are kept suspended and dispersed in the baths by means of mechanical or air agitation. In general, air agitation is preferred, and the air must not be contaminated with oil. The concentrations of powder may be as low as 1 g./l. and in some cases, as high as 500 g./l., though usually optimum results in codeposition are reached at concentrations of about 25 to 150 g./l. It is important that the fine particles are clean and wetted by the bath. For example, some commercial grades of tale (a hydrous magnesium silicate) must be washed with alcohol or acetone before they are readily wetted by the bath. In general, the particle sizes may be from about 10 microns down to 0.01 micron for inorganic particles, with the preferred range at about 5 microns down to 0.01 micron. With particles much greater than about 10 microns, roughness from the inorganic particles is obtained on areas on which settling can occur. Some agglomerated powders may have apparent larger particle size than the preferred size, but with agitation in the copper bath, the larger agglomerates are usually broken down and particles of about 5 microns diameter and under may then be the predominant size. With organic resin powders, the particle size may be as high as 50 microns size and still codeposit smoothly on vertical surfaces.

The pH of the acid copper plating baths unlike with nickel, does not have a profound effect on the percentage of the powder codeposited in the copper matrix. An acid copper sulfate bath having or 200 g./l. of sulfuric acid and low or high concentrations of copper sulfate,

yields in general very similar percentages of codeposition This is also true for acidic copper plating baths such as copper sulfamate, opper methane sulfonate, copper ethane sulfonate, and copper fluoborate. v To study the effect of pH on thefpercent inclusion of powder in the copper' 'plate from a sulfate bath, an acid copper sulfate bath was prepared with 190 g./l. of

hydrated copper sulfate and 35 g,/1. of boric acid. To this solution was added '5 g.'/l. of thallium ion by the addition of thallium sulfate to this bath, followed by the addition of 150 g./ l. of fine (about 0.1 to 3 microns size particles) of barium sulfate powder. The solution was air agitated to keep the powder dispersed in the bath. Table I shows the effect of the pH on the percent inclusion of powder in the copper plate.

It can be seen that after a pH of about 2.7, the percentage of inclusion does not increase. The same results were obtained when cesium sulfate was used instead of thallium sulfate. If higher concentrations of these monovalent cations are used, for example, 10 g./l., then as high as 5 to 5.5% by weight of barium sulfate is codeposited. Thallium and cesium are more effective than rubidium for obtaining the highest percentage of codeposition.

Variation in the temperature of the acid copper plating bath does exert a pronounced influence on the amount of powder which is codeposited with the copper plate. At room temperature there is a high amount of powder codeposited with the copper, but as the temperature of the bath is increased, less and less of the powder is included in the copper plate. To study the effect of temperature on the percent inclusion of the powder in the copper plate, an

acid copper sulfate bath was prepared with 190 g./l. of

hydrated copper sulfateand 60 g./l. of sulfuric acid. To this bath was added 5 g./l. of thallium ion in the form of thallium sulfate, and this was followed by the addition of 150 g./l. of fine barium sulfate powder. The bath was air more specifically illustrate the deposition ofthe 2-phase compositeplate."" i EXAMPLE I CuSO .5H O Concentrations in H 80 grams per liter Thallium ion (added as TI SO 100-250. BaSO -fine powder (0.1;to 5 mi- 0-100.

crons particle size) 0.1-10.

Temperature-60 to 140 F.

. 10-500. Current density Agitation-air or a mechanical.

10 to 100 amps/sq. ft.

EXAMPLE II cusoisn' o Conc. in g./l. H 50, 100-250.

Cesium ion (added as Cs SO 0-100.

B2180 fine powder (X-ray 0.1-100.

grade) Temperature60 to 140 F. 10-100.

I Agitation-air or Current density mechanical.

10 to 100 amps/sq. ft.

In Examples I and II, the weight percent of bariumsulfate'codeposited on vertical surfaces reached a maximum of about 5.5% As in Examples I and II, instead of barium sulfate powder, cerium oxide fine powder was used in concentrations of 10-500 g./l. and the weight percent codeposited on a vertical surface was about 4.7%.

EXAMPLE III Concentration in grams per liter CuSO SH O 100-250. H 80 0-100. Thallium (added as T1 SO 0.1-10. Fine boron powder 10-500. Temperature60 to 140 F. Agitation-air or mechanical. Current density 10 to 100 amps/ sq.ft.

Instead of fine, boron powder, fine zirconium oxide powder was used in concentrations of l-300 g./l., and the maximum weight percent co-deposited on a vertical suragitated to disperse the powder throughout the plating solution. The variation of percent inclusion of'the powder with variation of bath temperature is shown in Table II.

TABLE H Temperature of the copper sulfate bath (5 g./l.tha'l1ous Wt. percent inclusion barium sulfate powder Without the monovalent thallium cation present, the particles codeposit in traces if at all on a vertical surface.

face was about 8%. With the boron powder it was around 2% and with fine titania powder, it was around 4%. In the case of'fine silicon carbide powder (0.1 to 7 microns size) in concentrations of 5 to 500 g./l., the maximum codeposition on vertical surfaces was around 4 to 5% This is true for most non-conducting fine particles in acid 0 copper sulfate plating baths. It is not true for graphite and molybdenum sulfide particles which codeposit'readily and tend to cause rough plate. Thus, the acid copper sulfate plating baths are very different than the mildly acidic The acid coppersulfate bath is the preferred acid copper bath to use, and below are listed a few'examples to using thallium ion or cesium ion as promoters. Thus, in the examples given above, the weight percent of particles codeposited reached values of from 2% to about 5.5 and in one case, with zirconia powder, a value as high as8% was obtained.

Surfactants and brightening addition agents may be present in the acidcopper baths. When air agitation is used, the surfactants should not cause over-foaming, and

preferably an eight carbon chain length surfactant should 1 and 2% are all that is needed in 'many cases.

The monovalent cations are most effective in aiding codeposition of the dispersed fine bath-insoluble, nonconductmg partlcles in the acid copper sulfate baths. In

acidic copper fluoborate baths, the codeposition of the same dispersed inorganic particles in general proceeds quite well, and often the addition of monovalent cations are not needed as with barium sulfate and strontium sulfate particles, but even in this case, the addition of the monovalent cations can maximize the codeposition rate. In the case of conducting and semi-conducting particles such as graphite and molybdenum sulfide, the codeposition proceeds very readily in the acidic copper sulfate plating baths, but even here there is help from the monovalent cations. The main trouble with these powders is, that it is difiicult to get very smooth plate with their codeposition, unless the finest particles of these powders are used, but with the addition of the monovalent cations higher codeposition rates are obtained with lower concentrations of the particles which results in less problems with roughness.

In some cases mixtures of particles for codeposition is desirable, for example, barium sulfate with strontium sulfate, mica with barium sulfate, etc. Also, barium or strontium sulfate particles help to obtain smoother deposits with molybdenum sulfide and graphite.

In the case of organic resin powders such as PVC, nylon, Saran, polyethylene, polystyrene, polycarbonate, ABS, polyphenylene oxide, etc. it is often helpful but not essential to use a surfactant such as sodium 2-ethyl hexyl sulfate in the air-agitated baths. With fluorocarbon resin particles such as Teflon or Kel-F, it is necessary that a fluorocarbon surfactant such as potassium perfluoro 2-ethyl hexyl sulfonate be present in the acid copper plating bath in order to obtain the codeposition of the fluorocarbon resins with the monovalent cations such as thallium, cesium, rubidium, etc. present in the acid copper plating baths as the codeposition promoters.

While concentrations of thallium ion as low as 0.03 gram/liter, cesium ion as low as 0.05 gram/liter, rubidium ion as low as 0.1 gram/liter, and ammonium, sodium, potassium, lithium ions as low as 2 grams/liter are effective in promoting codeposition of the particles, higher concentrations are usually preferred, and, in fact, concentra tions of these ions at least as high as the copper ion concentration can be used. In general, with the most effective of the monovalent cations, thallium, cesium and rubidium concentrations of these ions higher than 5 to 15 grams/ liter are not needed.

It is to be appreciated that as used herein, the term Saran is intended to refer to polyvinylidene chlorides; the term nylon is intended to refer to polyamides; and the terms Teflon and Kel-F are intended to refer to fluorocarbon resins, such as the polytetrafluoroethylenes.

What is claimed is:

1. A method for electroplating which comprises codepositing onto a vertical cathode copper with fine bath insoluble substantially non-conducting particles dispersed in an agitated aqueous acidic copper electroplating bath which contains dissolved therein additionally at least one monovalent cation above copper in the electromotive series, and selected from the group consisting of thallium ion in concentrations of at least about 0.03 gram/liter, cesium ion in concentrations of at least 0.05 gram/liter, rubidium ion in concentrations of at least about 0.1 gram/ liter, sodium ion in concentrations of at least about 2 least 2 grams/liter, and ammonium ion in concentrations of at least about 2 grams/liter.

2. The method of claim 1 wherein the monovalent cation is present in the bath up to 15 grams/liter.

3. The method of claim 2 wherein the particles are (a) inorganic particles having a size ranging from about 0.1 to about 10 microns or (b) organic particles having a size ranging from about 0.1 to about 50 microns.

4. A method in accordance with claim 1 wherein said bath-insoluble particles are barium sulfate particles in concentrations of at least about 1 gram/liter and of particle size less than about 5 microns.

5. A method in accordance with claim 1 wherein said bath-insoluble particles are boron particles in concentrations of at least about 1 gram per liter and particle size less than about 5 microns.

6. A method in accordance with claim 1 wherein said bath-insoluble particles are silicon carbide particles in concentrations of at least about 1 gram/liter and particle size less than about 7 microns.

7. A method in accordance with claim 1 wherein said bath-insoluble particles are aluminum oxide particles in concentrations of at least about 1 gram/liter and particle size less than about 5 microns.

8. A method in accordance with claim 1 wherein said bath-insoluble particles are polyvinylchloride (PVC) particles in concentrations of at least about 1 gram/liter and particle size less than about 50 microns.

References Cited UNITED STATES PATENTS OTHER REFERENCES Kirk-Othmer Encyclopedia of Chemical Technology, vol. 8, 1965, pp. 30-32.

Transactions of the Electrochem. Soc., by Fink et al., 54, 1928, pp. 315-320.

GERALD L. KAPLAN, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204-181, 16 

